sub is_const
{use B;B::SVf_READONLY & B::svref_2object(\$_[0])->FLAGS}

       my $m="va"; our $n=3; my $u=undef; our $u2=undef;
       for $x ($m, $n, $u, $u2, 4, "a", undef) 
       { print "$x ";$x="VAR" unless is_const($x); print $x,"\n\n" }